The build quality on those old Nikkors was great. A couple weeks ago I was given a pretty trashed out 28 f2 from about 1973. The finish is almost completely worn off from decades of newspaper use and abuse. I think it was finally abandoned only because the auto aperture mechanism in the mount was frozen solid (moisture damage I think). I freed up the mechanism and shortened and reconnected the rusted through return spring. Amazingly it works great again even with extensive "cleaning marks" front and rear. The focus is silky smooth like you describe. I don't know the reason Nikon decided to make them this way, but they are less bothered by cold temps and decay of lubes than other lenses.